A wet electrostatic precipitator is a unit that is built into a conduit segment of a gas passage and separates finely divided solid or liquid particles from a gas stream or aerosol stream. Devices of this kind are therefore an indispensable component in production sectors of many kinds.
The process for separating the finely divided particles from the gas stream includes the following steps:
electrostatically charging the particles;
collecting the charged particles on the surface of an electrode or electrodes;
removing the charged particles from the surface of the collecting electrodes.
Electrostatic purification of an aerosol, i.e. finely divided particles in a gas stream, is usually achieved by way of negatively or positively charged particles (ions). They are generated by corona discharge, and become an actual electrical current because of the air gap between an electrode that is at an electrically positive or negative reference potential (usually ground potential) and a negative ionization electrode that is at an opposite electrical potential. These electrodes are connected to a high voltage source of the requisite polarity that supplies a direct current. The value of the applied voltage depends on the spacing between the electrodes and on the properties of the gas stream to be processed.
The efficiency of an electrostatic precipitator depends, over a wide range, on the intensity of the charge delivered to the particles by the charging segment. The charge intensity can be raised by increasing the electrostatic field in the ionization segment of the precipitator. The usual maximum intensity of the electrostatic field is limited to, at most, the value at which flashovers begin.
In wet electrostatic precipitators, the ionization and collection zones are combined in one unit. The collector tubes are often long, and therefore cause problems with adjustment of the discharge electrodes. Corona discharge stability in the ionization regions is also often influenced by the washing or rinsing (with water) of the internal surface of the collector tubes. These problems are described in DE 101 32 582 C1 and DE 102 44 051 C1, where the wet electrostatic precipitator is made up of a separate ionization region and collection region. The particles are charged by corona discharge in an intense electrostatic field. The corona discharge occurs in the gap between needle or star electrodes and the openings or nozzles of the grounded plate, when the needle or star electrodes are or become connected to DC high voltage. Oriented to the direction of the gas flow, the discharge electrodes project from downstream into the openings or nozzles of the grounded plate. The charged particles are collected in the grounded tube-bundle collector that follows the high-voltage electrodes downstream and is installed downstream from the ionization device.
A configuration of the wet electrostatic ionization stage is described in DE 101 44 051. The stage includes a plate, connected to ground potential or to a positive reference potential or counter-potential, that is installed across the open cross section of a flow conduit segment and has a plurality of identical openings through which the gas to be purified can flow. It is followed downstream by a high-voltage grid that is installed in electrically insulated fashion across the open cross section of the conduit segment, and is connected to a high-voltage potential via a passthrough in the wall of the conduit segment. A plurality, corresponding to the openings, of rod-shaped high-voltage electrodes are mounted at one end on this high-voltage grid and aligned. These high-voltage electrodes each point or project with their free end, identically and centrally, into an opening or nozzle of the plate.
A disk made of electrically conductive material, or at least coated therewith, sits in electrically connected fashion at each free end of a high-voltage electrode of this kind, centrally and parallel to the plate without touching it. Said disk has, evenly distributed around the circumference, at least two radial protrusions or tips that are directed radially or slightly outward, tilted toward the gas stream.
Operation of the wet electrostatic precipitator shows that increasing the applied voltage, which means increasing the electrical field strength in the electrode gap, provokes a spark discharge that occurs in a manner corresponding to the non-homogeneous electric field between the electrodes and the edges of the openings or nozzles. This decreases particle charging efficiency and particle collection efficiency in the electrostatic precipitator.
DE 10 2005 023 521 describes a wet electrostatic ionization stage in an electrostatic precipitation device for removing, from an aerosol, i.e. a gas, finely divided particles that are also transported in the gas. It comprises a plate, connected to ground potential or to a referred counter-potential, that is installed across the open cross section of a flow conduit segment and has a plurality of identical openings through which the gas to be purified can flow. The ionization stage has a high-voltage grid that is installed in electrically insulated fashion across the open cross section of the conduit segment, downstream or upstream in the gas flow with respect to the plate, and is connected to a high-voltage potential via a passthrough in the wall of the conduit segment. It additionally has a plurality, corresponding to the openings or nozzles) of rod-shaped high-voltage electrodes that are mounted at their one end on the high-voltage grid and each project with their free end, in identically central fashion in each case, into a nozzle of the nozzle plate. At these free ends, identically in each case, a disk made of electrically conductive material sits centrally and parallel to the plate without touching it. A disk has, evenly distributed around its circumference, at least two radial outward protrusions or tips.
Depending on overall size and electrical potential conditions, the distance D between the high-voltage grid and the end face of the sleeves that faces it is at least such that the possibility of spark discharge between these two physical assemblies during operation of the precipitator is ruled out. This is a high-voltage engineering design that accounts for the process environment.
Inserted identically into each nozzle having a simple convex round or polygonal open cross section is a sleeve of similar cross section whose axis is perpendicular to the plate that is at reference potential (often ground potential). In consideration of operating conditions, in particular the intensity of the gas flow, the sleeve often also sits in frictionally engaged fashion to neutralize normal operating influences, and because of the provision of maintenance work is mounted or positioned releasably in the nozzle.
The disk is exposed, inside the sleeve, at the free end of this rod-shaped high-voltage electrode. A simple convex round or polygonal enveloping curve of the disk has, circumferentially, a constant spacing L with respect to the sleeve. In the gap between the inner wall of the sleeve and the rim of the disk, the electrical potential difference is made up of the high-voltage potential and reference or ground potential.
For efficient long-term operation of the electrostatic precipitator, it is important that the electrical conditions be maintained or can be maintained. This means that, in particular, the geometry established between the nozzle plate and the high-voltage electrodes positioned at it remain unchanged in order to limit electrical flashovers and suppress high-current discharges.
With the construction of the ionization stage as described in DE 10 2005 023 521, upon extended operation a deposition of particles on the impermeable sleeve wall was unavoidable; this then modified the electrical situation in the gaps between the nozzles or sleeves and the respectively associated high-voltage electrodes disadvantageously (in the direction of more flashovers), and resulted ultimately in loss of effectiveness due to short circuit.